Christopher Keefe, 28, of Braintree is facing arraignment in Quincy District Court on charges of assaulting an elderly person.

Both also received citations for fare evasion, said Detective Richard Sullivan, a Transit Police spokesman.

Hamilton was stopped by Transit Police officers after they allegedly observed him “piggybacking” behind a paying customer at the MBTA Massachusetts Avenue station at about 6:30 p.m. Monday.

Keefe was allegedly seen by police attempting to evade the fare 20 minutes later at the same station. Plainclothes Transit Police officers allegedly saw him telling a customer service agent he was an “active duty” member of the US military and wanted to ride the T free of charge.

The officers asked Keefe to show them his military ID. He could not produce one. The officers ran his name through their computer records and discovered he had an outstanding arrest warrant for allegedly assaulting an elderly person in Quincy. Keefe was not in the military, Transit Police said.